In a fuel cell power plant comprising a re-circulation passage which re-circulates anode effluent discharged from an anode into the hydrogen that is supplied to the anode, impurity gases such as nitrogen become mixed into the anode gas as an operation of the fuel cell power plant progresses. Such impurity gases cause a reduction in the hydrogen partial pressure within the anode gas, thus leading to deterioration in the output performance of the fuel cell stack.
JP2000-243417A, published by the Japan Patent Office in 2000, discloses a method of removing impurity gases by purging the gas in the re-circulation passage in accordance with a decrease in the hydrogen concentration of the anode gas or a decrease in the output of the power plant.
JP2001-006709A, published by the Japan Patent Office in 2001, discloses a method of preventing poisoning of the anode caused by carbon monoxide (CO) contained in the anode gas by mixing a small amount of oxygen into the anode gas.